Question: Which of these numbers is prime? ${3,\ 40,\ 49,\ 55,\ 93}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 40 are 1, 2, 4, 5, 8, 10, 20, and 40. The factors of 49 are 1, 7, and 49. The factors of 55 are 1, 5, 11, and 55. The factors of 93 are 1, 3, 31, and 93. Thus, 3 is a prime number.